1. Field of the Invention
The present invention relates to the analysis of downhole borehole fluids. More particularly, the present invention relates to apparatus and methods for the in situ determination of gas-oil ratio of fluids in a geological formation.
2. State of the Art
Those skilled in the art will appreciate that the ability to conduct an analysis of formation fluids downhole (in situ) is extremely desirable. With that in mind, the assignee of this application has provided a commercially successful borehole tool, the MDT (a trademark of Schlumberger) which extracts and analyzes a flow stream of fluid from a formation in a manner substantially as set forth in co-owned U.S. Pat. Nos. 3,859,851 and 3,780,575 to Urbanosky which are hereby incorporated by reference herein in their entireties. The OFA (a trademark of Schlumberger), which is a module of the MDT, determines the identity of the fluids in the MDT flow stream and quantifies the oil and water content based on the previously incorporated related patents. In particular, previously incorporated U.S. Pat. No. 4,994,671 to Safinya et al. provides a borehole apparatus which includes a testing chamber, means for directing a sample of fluid into the chamber, a light source preferably emitting near infrared rays and visible light, a spectral detector, a data base means, and a processing means. Fluids drawn from the formation into the testing chamber are analyzed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information accordingly (and preferably based on the information in the data base relating to different spectra), in order to quantify the amount of water and oil in the fluid. As set forth in previously incorporated U.S. Pat. No. 5,266,800 to Mullins, by monitoring optical absorption spectrum of the fluid samples obtained over time, a determination can be made as to when a formation oil is being obtained as opposed to a mud filtrate. Thus, the formation oil can be properly analyzed and quantified by type. Further, as set forth in the previously incorporated U.S. Pat. No. 5,331,156 to Hines et al., by making optical density measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified.
While the Safinya et al., Mullins, and Hines et al. patents represent great advances in downhole fluid analysis, and are particularly useful in the analysis of oils and water present in the formation, they do not address in detail the gases which may be plentiful in the formation. The issue of in situ gas quantification is addressed in the previously incorporated U.S. Pat. Nos. 5,167,149 to Mullins et al., and 5,201,220 to Mullins et al., and in O. C. Mullins et al., "Effects of high pressure on the optical detection of gas by index-of-refraction methods", Applied Optics, Vol. 33, No. 34, pp. 7963-7970 (Dec. 1, 1994) which is also incorporated by reference herein in its entirety, where a rough estimate of the quantity of gas present in the flow stream can be obtained by providing a gas detection module having a detector array which detects light rays having certain angles of incidence. While rough estimates of gas quantities are helpful, it will be appreciated that more accurate measurements are often necessary.
One particularly important measurement for newly discovered oil is the gas-oil ratio (GOR). The GOR is conventionally defined as the volume of gas at STP (standard temperature and pressure) in cubic feet divided by the number of stock tank barrels of oil in a quantity of formation fluid. A GOR of 6,000 ft.sup.3 /bbl represents approximately equal mass fractional amounts of gas and oil. The GOR must be known in order to establish the size and type of production facilities required for processing the newly discovered oil. For example, a very large GOR of approximately 11,000 ft.sup.3 /bbl will require the construction of expensive gas handling facilities. It is therefore important to make an accurate measurement of GOR in newly discovered oil so that the appropriate financial investment in production facilities is made.
Currently, the most accurate and preferred method of establishing GOR is to take several samples of formation fluid and subject the samples to laboratory analysis. It is understood that the samples taken must be an accurate statistical representation of the formation fluid in order for the analysis to provide accurate results. In order to enhance the accuracy of the laboratory analysis, many samples are taken from different locations in the formation. However, sample collection of high GOR fluids can be very difficult as samples are not valid if phase separation occurs during sampling. Furthermore, in the process of shipping gas containing samples and performing laboratory analysis, gas can leak from the containers and ruin the samples.
Recently, downhole fluid analysis has been used, providing rough estimates of GOR, in order to aid in the selection of samples for laboratory testing. Theoretically, if estimates of GOR at several locations in the formation are the same or similar, a single fluid sample may be sufficient to provide an accurate laboratory analysis of GOR. One of the recently used methods for estimating GOR relies primarily on the coloration of the fluid sample. Lighter color oils tend to have high gas fractions, generally, although not necessarily indicating a larger GOR. Moreover, this method is indirect and prone to error because the coloration measures the heavy aromatic content whereas, for GOR, one actually wants to measure the methane and light hydrocarbon fractions.
Co-owned U.S. Pat. No. 4,994,671 discloses an apparatus and method for analyzing the composition of formation fluids through the use of spectroscopy. Spectroscopy has been used downhole for distinguishing between oil and water (in the near infrared spectrum), and for distinguishing among oils (in the visible spectrum). However, for several reasons, downhole spectroscopy has not been suggested for distinguishing between gas and oil or for distinguishing among different hydrocarbon gases such as methane (CH.sub.4), ethane (having methyl components (CH.sub.3)), and higher hydrocarbons which contain predominantly methylene (CH.sub.2). First, because the density of a gas is a function of pressure, and because downhole pressures can vary by a factor of thirty or more, the dynamic range of the gas densities likely to be encountered downhole is extremely large. As a result, it is believed that the dynamic range of the spectral absorption at frequencies of interest is also extremely large such as to make a measurement unfeasible; i.e., the sensitivity of the downhole spectroscopy equipment is typically incapable of handling the large dynamic ranges that are expected to be encountered. Second, due to fact that the condensed phase of hydrocarbon (oil) has a much higher density at downhole pressures than the gas phase, it is believed that a thin film of liquid oil on the OFA window can yield significant absorption. Thus, where an oil film was present, interpretation of the results would yield a determination of a rich gas mixture, where no or little amount of hydrocarbon gas was actually present. Third, the type of spectral analysis typically done uphole to distinguish among hydrocarbon gases cannot be done downhole. In particular, in uphole applications, individual gas constituents are detected by modulating a narrow band source on and off of mid-infrared absorption lines of the gas, where a resulting oscillation in absorption at each modulation frequency would indicate a positive detection of a particular gas. However, at the high pressures encountered downhole, not only are the narrow gas absorption spectral lines merged, but mid-infrared spectroscopy is hindered by the extreme magnitude of the absorption features. Fourth, spectrometers are typically sensitive to changes in temperature, and elevated temperatures encountered downhole can induce spectral changes of the gas sample, thereby complicating any data base utilized.
Co-owned application Ser. No. 08/827,647 now U.S. Pat. No. 5,859,430 discloses a method and apparatus for the downhole compositional analysis of formation gases which utilizes a flow diverter and spectrographic analysis. More particularly, the apparatus includes diverter means for diverting formation gas into a separate stream, and a separate gas analysis module for analyzing the formation gas in that stream. By providing a diverter means and a separate gas analysis module, the likelihood of a having a thin film of oil on the cell window is decreased substantially, thereby improving analysis results. Also, by providing one or more cells with different path lengths, issues of dynamic range are obviated, because where the pressure is higher, light will not be fully absorbed in the cell having a short path length, whereas where the pressure is lower, there will be some absorption in the cell having the longer path length. The methods and apparatus of the '647 application are useful in determining what types of gas are present in the formation fluid, but are not particularly useful in determining GOR.